This invention relates to an adaptive braking system for a fluid pressure operated vehicle braking system.
Governmental regulations for vehicles having brakes operated by air pressure require rapid brake actuation response during brake application and consequently require components which have relatively large fluid pressure flow rates. Although these high fluid pressure flow rates are desirable during normal brake operation, they are undesirable when adaptive control of the vehicle brakes is necessary, since the relatively high flow rates during brake pressure increase cycles of the adaptive braking system may cause the vehicle's brakes to cause the wheels to lock, thereby extending the stopping distance, because the adaptive braking system cannot respond quickly enough to prevent wheel lock in braking systems with the aforementioned high pressure flow rates. Accordingly, it is desirable to limit the brake pressure increase during pressure increasing cycles when the vehicle's brakes are controlled by the vehicle adaptive braking system. Prior art adaptive braking systems have overcome this problem by employing a duty cycle translator which converts a brake pressure control signal into a prescribed duty cycle for a solenoid actuated valve which is a part of the modulating relay valve controlling communication to the brake actuators. In this type of system, the solenoid is operated at a sufficiently high frequency that the duty cycle is integrated into a steady state pressure level in the vehicle brake actuators. Unfortunately, these types of systems are characterized by their slow response, which is detrimental to good adaptive braking performance, because they are open-loop systems. Theoretically, this defect may be cured by providing a pressure feedback signal by locating a pressure transducer in the modulating relay valve or brake pressure actuators and by comparing the signal generated by the transducer to the brake pressure control signal. However, pressure transducers increase system cost substantially, and complicate installation problems.
U.S. Pat. No. 3,953,083 (Latvala et al), owned by the assignee of the present invention and incorporated herein by reference, discloses an adaptive braking system having a duty cycle translator which also includes a pseudo-feedback signal generating circuit which generates a feedback signal approximating the brake pressure level in the vehicle brake actuators without direct measurement of this pressure level. Clearly, the response time of any adaptive braking system includes the opening and closing times of the solenoid actuators used to control pressure communication to the vehicle brakes. However, these solenoids are operated by power from the vehicle electrical system, the voltage of which may vary over a relatively wide range depending upon condition of vehicle battery, condition of the charging system, electrical load, ambient temperature conditions, etc. Also these solenoids operate at various fluid pressure levels. The present invention discloses an improvement of the invention disclosed in the Latvala et al patent discussed hereinabove, in which the pseudo-feedback circuit includes circuits which model the opening and closing times of the solenoid actuators, which includes a circuit which models the time delays due to current build-up and relay in the solenoid coil and further includes a circuit which models the time delays due to pressure levels at the solenoid. Consequently, the adaptive braking system disclosed herein generates a pseudo-feedback signal which approximates the brake pressure level in the vehicle brake actuators very accurately.